1. Field of the Invention
The present invention relates to cables, in particular for low-voltage electrical energy distribution or for telecommunications, these cables having low-smoke self-extinguishing properties, and to the flame-retardant compositions used therein.
2. Description of the Related Art
Self-eixtinguishing cables can be produced having a flame-retardant coating made from a polymer composition to which fire-resistant properties have been given by adding a suitable additive. Polyolefin-based compositions based, for example, on polyethylene or ethylene/vinyl acetate copolymers, containing an organic halide combined with antimony trioxide as flame-retardant additive can, for example, be used for this purpose. However, halogenated flame-retardant additives have many drawbacks since they partially decompose during processing of the polymer, giving rise to halogenated gases that are toxic to workers and corrode metal parts of the polymer-processing equipment. In addition, when they are placed directly in a flame, their combustion gives rise to large amounts of fumes containing toxic gases. Similar drawbacks are encountered when polyvinylchloride (PVC) supplemented with antimony trioxide is used as base polymer.
Therefore, in recent years the production of self-extinguishing cables has been directed toward halogen-free compositions, using as flame-retardant filler inorganic oxides, preferably in hydrate or hydroxide form, in particular magnesium hydroxide or aluminium hydroxide.
Aluminium hydroxide starts to decompose at a relatively low temperature (about 190xc2x0 C.), which can result in various drawbacks during extrusion of the polymer composition, with formation of bubbles and defects in the final product. Therefore, the use of aluminium hydroxide as flame retardant is generally limited to polymer materials which do not require high processing temperatures. In contrast, magnesium. hydroxide has a decomposition temperature of about 340xc2x0 C. and is characterized by greater heat stability and a high decomposition enthalpy. These properties make magnesium hydroxide particularly suitable as flame retardant filler in polymer compositions for coating cables, which require high extrusion temperatures and a small number of morphological defects.
However, the use of magnesium hydroxide as a flame-retardant filler does have certain drawbacks. Firstly, in order to obtain an efficient flame-retardant effect, very large amounts of magnesium hydroxide must be added to the polymer material, generally about 120-250 parts by weight relative to 100 parts by weight of polymer material. Such high levels of filler lead to a reduction in processability and in mechanical and elastic properties of the resulting mixture, in particular as regards impact resistance, elongation and stress at break.
In the U.S. Pat. No. 4,145,404 these drawbacks are attributed to the low affinity of natural magnesium hydroxide, obtained for example by grinding minerals such as brucite, with the polymer material, in particular when the polymer is of low polarity, as in the case of polyolefins.
In the patent EP-780,425 it is pointed out that the presence of different metal impurities, such as iron or manganese salts, in magnesium hydroxide of natural origin causes degradation of the polymer matrix into which the magnesium hydroxide is inserted.
Therefore, research efforts have been directed towards modifying properties of magnesium hydroxide to improve its compatibility with the polymer matrix and its degree of purity. Various synthetic methods have thus been developed in which magnesium hydroxide is produced by adding alkalis to an aqueous solution of a soluble salt thereof and subsequent precipitation. of the hydroxide by heating at high pressure (see for example patent U.S. Pat. No. 4,098,762 or the above-mentioned patents EP-780,425 and U.S. Pat. No. 4,145,404). In this way, a magnesium hydroxide is obtained with a high degree of purity and high structural uniformity with formation of crystallites of flattened hexagonal shape with an average diameter not greater than 2 xcexcm and a specific surface area, measured by BET method, not greater than 20 m2/g.
However, the use of synthetic magnesium hydroxide as flame-retardant filler has a considerable impact on the cost of the finished product, so as to make flame-retardant systems based on magnesium hydroxide non-competitive when compared with the halogen-containing flame-retardant compositions described above.
In certain cases attempts have been made to improve properties of natural magnesium hydroxide using suitable grinding and/or surface treatment processes.
For example, Japanese patent application JP-01-294792 (Kokai) describes a process for the production of magnesium hydroxide, in which natural brucite is wet-ground so as to obtain an average particle diameter of between 2 and 6 xcexcm, and then surface-treated with a fatty acid ammonium salt, and eventually dried. The resulting magnesium hydroxide would be resistant to efflorescence phenomena caused by carbonation of magnesium hydroxide by atmospheric carbon dioxide. The process of wet-grinding is considered essential to make the particle size of the product more uniform without increasing its lattice distortion coefficient which is thought to be responsible for high resistance to carbonation of natural magnesium hydroxide. The surface treatment is thought to improve dispersibility of the filler in the polymer matrix. The magnesium hydroxide thus obtained is claimed to be useful as a flame-retardant for polyolefin resins. In particular, the examples describe compositions with flame-retardant properties based on ethylene/vinyl acetate (EVA) and ethylene/ethyl acrylate (EEA) copolymers.
Japanese patent application JP-03-231,944 (Kokai) describes polyolefin-based compositions having flame-retardant properties and containing magnesium hydroxide with an average particle diameter of between 3 and 13 xcexcm and the following particle size distribution: 1-20% by weight of particles with a diameter less than or equal to 1 xcexcm; 55-98% by weight of particles with a diameter between 1 and 15 xcexcm; 1-25% by weight of particles with a diameter between 15 and 50 xcexcm. This particle size distribution is believed to afford higher flame resistance, which would be accompanied by good mechanical strength, flexibility and processability. A magnesium hydroxide with these properties would be obtainable by suitable grinding of natural brucite, followed by sieving or addition of another material of predetermined particle size. According to the description given in the above-mentioned patent application, this type of magnesium hydroxide would be useful as a flame-retardant filler for polyolefins such as polyethylene, olefinic rubbers, polypropylene, polybutene and the like. Particular mention is made of ultra-low-density polyethylene (ULDPE) having a density of 0.860-0.910 g/cm3, obtainable by copolymerization of ethylene with an alpha-olefin in the presence of a conventional Ziegler-Natta catalyst based on titanium and/or vanadium compounds.
Lastly, Japanese patent application JP-05-17692 (Kokai) describes polymer compositions having flame-retardant properties and containing natural magnesium hydroxide which has previously been ground and surface-treated with a fatty acid or a fatty acid salt, or alternatively with a silane or a titanate acting as coupling agent. These compositions would be characterized by high resistance to acid attacks. The subsequent Japanese patent application JP-07-161230 (Kokai) describes compositions similar to the above, pointing out that, in order to decrease the hygroscopicity of magnesium hydroxide, the latter must be surface-treated with the same products as mentioned above, in amounts of between 0.5 and 5% by weight relative to the magnesium hydroxide weight. In both of the above-mentioned Japanese patent applications, polyolefins such as polyethylene, ethylene/propylene rubbers, acrylic rubbers and the like are cited as polymeric materials, and flame-retardant compositions based on ethylene/ethylacrylate (EAA) polymers are given as particular examples. No information is provided regarding mechanical, elastic or processability properties of the resulting mixtures.
From the foregoing, it is clear that in the prior art considerable efforts have been made to improve the properties of flame-retardant polymer compositions containing magnesium hydroxide by modifying the properties of magnesium hydroxide itself, in terms of crystallinity, particle size distribution and/or surface properties. These modifications have been achieved either by developing synthetic processes starting from soluble magnesium salts or by appropriately modifying and treating natural magnesium hydroxide. For the purposes of the present invention, with enhanced flame-retardant properties it is meant that a cable passes a test as defined by standard CEI 332-1; with enhanced mechanical properties it is meant a high elongation at break value and a relatively low modulus, which are capable of determining a cable flexibility which is suitable for use; in particular, it is meant that mechanical properties are essentially not lower than those of cables using compositions of known type, for example halogenated compositions.
The Applicant has now found that it is possible to produce self-extinguishing, halogen-free cables producing a low level of fumes and having high flame resistance and excellent mechanical performances by using natural magnesium hydroxide as flame-retardant filler and, as polymer matrix, a polymeric mixture comprising a crystalline propylene homopolymer or copolymer and a copolymer of ethylene with an alpha-olefin, and optionally with a diene, characterized by uniform distribution of the alpha-olefin among the copolymer molecules.
Therefore, according to a first aspect, the present invention relates to a cable with self-extinguishing properties, comprising a conductor and a flame-retardant coating, characterized in that the said flame-retardant coating comprises:
(a) a crystalline propylene homopolymer or copolymer;
(b) a copolymer of ethylene with at least one alpha-olefin, and optionally with a diene, said copolymer (b) being characterized by a composition distribution index greater than 45%, said index being defined as the weight percentage of copolymer molecules having an alpha-olefin content within 50% of the average total molar content of alpha-olefin;
(c) natural magnesium hydroxide in an amount such as to impart flame-retardant properties.
In a second aspect, the present invention relates to a flame-retardant composition comprising:
(a) a crystalline propylene homopolymer or copolymer;
(b) a copolymer of ethylene with at least one alpha-olefin, and optionally with a diene, said copolymer (b) being characterized by a composition distribution index greater than 45%, said index being defined as the weight percentage of copolymer molecules having an alpha-olefin content within 50% of the average total molar content of alpha-olefin;
(c) natural magnesium hydroxide in an amount such as to impart flame-retardant properties.
The composition distribution index provides an indication of the distribution of the alpha-olefin among the copolymer molecules (the higher the value of this index, the more homogeneous the distribution of the comonomer among the copolymer molecules), and can be determined by Temperature Rising Elution Fractionation, as described, for example, in patent U.S. Pat. No. 5,008,204 or in Wild et al., J. Poly. Sci. Poly. Phys. Ed., Vol. 20, p. 441 (1982).
In the Applicant""s view, the composition distribution index is related to the ability of the copolymers of ethylene with an alpha-olefin, and optionally with a diene, to incorporate and disperse large amounts of the flame-retardant filler, thereby obtaining a mixture having excellent flame-resistance and, at the same time, good processability and improved mechanical properties. Given a certain ratio between flame-retardant filler and polymer matrix, it is important to determine the minimum value of this index which is sufficient to obtain the desired combination of mechanical properties and processability.
Moreover, the presence in the polymer mixture of a crystalline propylene homopolymer or copolymer makes it possible to obtain a thermoplastic coating which has increased thermocompression resistance even at the maximum operating temperatures, so as to pass the thermocompression test described in CEI standard 20-34/3-1. This test consists in subjecting the coating of a cable specimen to a predetermined compression at a predetermined temperature and for a predetermined time. At the end of the test, the flattening degree of the coating, expressed as percentage of the residual thickness relative to the initial thickness of the coating, is measured: the sample passes the test if its residual thickness is greater than 50% of its initial thickness.
In a further aspect, the present invention relates to a method for obtaining a cable having improved mechanical properties and increased fire resistance, said method comprising the following steps: (1) preparing a polymer mixture having flame-retardant properties; (2) extruding said mixture on a conductor optionally pre-coated with an insulating layer, characterized in that step (1) comprises mixing a predetermined amount of natural magnesium hydroxide with a polymer mixture comprising:
(a) a crystalline propylene homopolymer or copolymer, as a polymeric component capable of increasing the thermocompression resistance of the flame-retardant coating; and:
(b) a copolymer of ethylene with at least one alpha-olefin, and optionally with a diene, capable of dispersing natural magnesium hydroxide, so as to improve processability of the mixture and enhance mechanical properties of the flame-retardant coating.
The amount of natural magnesium hydroxide to be added is predetermined so as to obtain a cable which is capable of passing the fire-resistance test according to CEI standard 332-1. The amount of propylene homopolymer or copolymer (a) is such that the flame-retardant coating obtained after extrusion has a value of thermocompression resistance, measured at 100xc2x0 C. according to CEI standard 20-34/3-1, greater than 50%. The amount of copolymer (b) is such that the flame-retardant coating obtained after extrusion has an elongation at break, measured according to CEI standard 20-34 xc2xa75.1, of at least 100%, preferably of at least 150%, and a modulus at 20%, measured according to CEI standard 20-34 xc2xa75.1, of less than 12 MPa, preferably less than 7 MPa.